Chemical treatment to improve red mud separation and washing in the bayer process

ABSTRACT

The invention provides methods and compositions for improving the rheology of red mud removed from Bayer Process liquor. The method includes adding a flocculant and a cross-linked polysaccharide to the liquor. This combination separates the red mud from the liquor but also prevents the red mud from becoming too thick. By preventing excessive thickness, the method allows for the formation of extremely dense amounts of red mud even in primary settlers because the dense red mud can still flow. As a result a user can simultaneously enjoy both easy handling of red mud and also high recovery rates of valuable alumina and caustic from the red mud.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of co-pending U.S. patentapplication Ser. No. 13/662,964 filed on Oct. 29, 2012 which in turn isa continuation in part of U.S. patent application Ser. No. 12/852,91.0filed on Aug. 9, 2010 and which has issued as U.S. Pat. No. 8,298,508.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

The invention relates to compositions, methods, and apparatuses forimproving the performance of primary settlers and washer circuits forred mud in a Bayer Process. As described for example in U.S. Pat. Nos.6,814,873, 6,033,579, 6,048,463, and US Published Patent Application2008/0257827, in the Bayer Process alumina trihydrate is extracted frombauxite ore. Bauxite ore is pulverized then goes through a digestionstage (where it is slurried in a caustic liquor medium and is heatedunder pressure), a clarification stage, a precipitation stage, aclassification stage, and finally a calcination stage from which thefinal product is recovered.

In the clarification stage red mud is separated from the alumina in theore. Red mud is the large percentage of the ore that remains insolubleafter digestion with caustic medium. This insoluble traction must beremoved prior to the alumina trihydrate recovery step in the process soas to avoid contaminating the final Bayer Process product. The digestionslurry consists of finely suspended mud particles which are commonlyremoved by the addition of flocculants in large separation vesselscalled thickeners or settlers. The flocculant acts to bind the mudparticles increasing their rate of settling in the thickener. Theoverflow liquor then typically reports to filters, often referred to asSecurity Filtration, to remove any remaining insoluble material prior toalumina trihydrate recovery. The red mud slurry from the thickenerunderflow contains valuable alumina and caustic in solution that isrecovered in the Red Mud Washer Circuit otherwise known as acounter-current decantation (CCD) circuit. Over several stages, mudslurry is mixed with progressively more dilute (lower in valuablealumina and caustic) wash water. The advancing wash water progressivelyincreases in caustic and alumina content as it recovers the valuablesfrom the mud slurry, then ultimately exits as a dilution stream for useback into the process at an appropriate point. The resulting red mud issent to disposal. Some examples of red mud clarification are describedin U.S. Pat. Nos. 3,085,853, 3,397,953, 3,445,187, 3,541,009, 3,681,012,4,767,540, and 5,008,089.

Because of the cost associated with each additional filtration anddilution step, large savings and efficiencies can be realized fromreducing the number or intensity of the required filtration and dilutionsteps. As a result, there is clear utility in novel methods of moreeffectively flocculating and separating red mud from the redmud-containing liquors.

The art described in this section is not intended to constitute anadmission that any patent, publication or other information referred toherein is “prior art” with respect to this invention, unlessspecifically designated as such. In addition, this section should not beconstrued to mean that a search has been made or that no other pertinentinformation as defined in 37 CFR§1.56(a) exists.

BRIEF SUMMARY OF THE INVENTION

At least one embodiment of the invention is directed towards improvingthe process of separating and washing insoluble red mud solids in theBayer process by addition of cross-linked polysaccharides in combinationwith flocculants in the primary settling and washing stages of theprocess.

DETAILED DESCRIPTION OF THE INVENTION

The following definitions are provided to determine how terms used inthis application, and in particular how the claims, are to be construed.The organization of the definitions is for convenience only and is notintended to limit any of the definitions to any particular category.

“Clarifier” means a separation device used for red mud clarificationincluding: a thickener, a settler, or a washer.

“Consisting Essentially of” means that the methods and compositions mayinclude additional steps, components, ingredients or the like, but onlyif the additional steps, components and/or ingredients do not materiallyalter the basic and novel characteristics of the claimed methods andcompositions.

“Dextran” is a polysaccharide characterized as being an α-D-1,6glucose-linked glucan with side chains 1-3 linked to the backbone unitsof the polysaccharide.

“Distal” is the opposite of “Proximal” and means subsequent to aparticular step in a sequential process.

“Flocculant” means a composition of matter which when added to a liquidcontaining finely divided suspended particles, destabilizes andaggregates the solids through the mechanism of interparticle bridging,it may have a low charge density and/or may have a high molecular weight(in excess of 1,000,000).

“Green Liquor” means the alumina containing liquor that has passedthrough a Security Filtration stage and no longer contains red mud.

“Liquor” or “Bayer liquor” means a caustic, liquid medium that has runthrough a Bayer process in an industrial facility.

“Polysaccharide” means a polymeric carbohydrate having a plurality ofrepeating units comprised of simple sugars, the C—O—C linkage formedbetween two such joined simple sugar units in a polysaccharide chain iscalled a glycosidic linkage, and continued condensation ofmonosaccharide units will result in polysaccharides, commonpolysaccharides are amylase and cellulose, both made up of glucosemonomers, polysaccharides can have a straight chain or branched polymerbackbone including one or more sugar monomers, common sugar monomers inpolysaccharides include glucose, galactose, arabinose, mannose,fructose, rahmnose, and xylose.

“Primary Settler Feed” means Bayer process digested slurry as charged tothe first solids/liquid separation stage, it may be an admixture of thedigested slurry plus dilution liquor, and the dilution liquor isroutinely the counter current technique wash water from the red mudwashing stages, the primary settler feed differs from the liquors orslurries subjected to clarification and/or separation in the subsequentclarification stage or the red mud washing stages by composition as tothe solids content, dissolved sodium alum mate content, and totalalkalinity, in addition, the primary settler feed also differs from theliquors or slurries subjected to clarification and/or separation in thesubsequent clarification stage or the red mud washing stages in that noinsoluble fraction thereof has received an earlier flocculationtreatment.

“Proximal” is the opposite of “Distal” and means prior to a particularstep in a sequential process.

“RedMud” means the insoluble solid material which is a residual productfrom the Bayer liquor that does not freely dissolve during the digestionstage, or which precipitates as part of the digestion process. Red mudcan comprise ferric oxide (from which the mud typically derives itsred-brown color), alumina, silica, silicon oxide, calcium oxide, sodiumalumino-silicates, and/or titanium oxides (depending on the specificcompositions of the ore input into the Bayer process) as well as causticand other materials from the digestion liquor that may be part of theliquid phase of the Bayer process red mud slurry.

“Rheology” means the interrelatedness of the flow rate and the elastic,viscous and/or plastic properties of flowing matter.

“Slurry” means a mixture comprising a liquid medium within which finesolid particles are dispersed or suspended.

“Thickener” or “Settler” means a vessel used to effect a solid-liquidseparation of a slurry, often with the addition of flocculants, thevessel constructed and arranged to receive a slurry, retain the slimyfor a period of time sufficient to allow solid portions of the slurry tosettle downward (underflow) away from a more liquid portion of theslurry (overflow), decant the overflow, and remove the underflow.Thickener underflow and thickener overflow are often passed on tofilters to further separate solids from liquids.

“Washer” means a vessel used to effect a solid-liquid separation of redmud from liquid by utilizing counter current, decantation separating thematerial into an underflow (a highly concentrated suspension typicallyat the bottom of the washer), and an overflow (a clarified liquid streamtypically at the top of the equipment), the liquid used to effect thecounter current may be water, liquor a mixture of water and liquor oroverflow from elsewhere in the Bayer Process.

In the event, that the above definitions or a description statedelsewhere in this application is inconsistent with a meaning (explicitor implicit) which is commonly used, in a dictionary, or stated in asource incorporated by reference into this application, the applicationand the claim terms in particular are understood to be construedaccording to the definition or description in this application, and notaccording to the common definition, dictionary definition, or thedefinition that was incorporated by reference. In light of the above, inthe event that a term can only be understood if it is construed by adictionary, if the term is defined by the Kirk-Othmer Encyclopedia ofChemical Technology, 5th Edition, (2005), (Published by Wiley, John &Sons, Inc.) this definition shall control how the term is to be definedin the claims.

At least one embodiment of the invention is directed towards a method oftreating Bayer Process red mud slurry to enhance the recovery of causticand alumina. The treatment can occur in settlers and In washer circuits.The method comprises contacting primary settler feed or washer feedslurry with a flocculant in combination with a modified polysaccharide.The method modulates the theology of thickened red mud allowing higherunderflow densities to be targeted in each vessel of the Wash Circuit orin the primary settlers.

Red mud slurry flow in the clarification stage suffers from a number ofcontradictory properties. Ideally the underflow of a settler or washershould have a very high density. Such a high density results from highlyeffective removal of soluble caustic and alumina from the red mudunderflow, so effective in fact that the red mud comprises onlyinsoluble materials. However as solids density in the underflowincreases, the yield stress of the slurry will also increase andconsequently this reduces the ability of the underflow to actually flowthrough the equipment used in the clarification process. As a result, inpractice the underflow density must be kept below a degree of thicknesseven though this means that valuable alumina and caustic remain withinthe underflow. This is a costly practice which reduces the amount ofactual alumina that can be recovered from a given sample of ore to anamount significantly below its theoretical yield.

Additionally, by reducing the level of suspended solids that remain inthe supernatant above the flocculated slurry formed in the primarysettler liquor, the solids to be removed during the subsequentclarification of the overflow by filtration are diminished.

In at least one embodiment the method alters the rheology (viscosity,elasticity, and/or plasticity) of the underflow that passes throughwasher(s) and/or settler(s) such that the underflow has properties thatwould otherwise be associated with an underflow slurry of lower solidsdensity. This allows operators to increase the underflow density in one,some, or all of the vessels of the circuit, resulting in betterextraction efficiency of alumina and caustic and increased throughput.

In at least one embodiment the underflow passing through at least aportion of the circuit has a density so high that for the method, itwould not be capable of passing through the washer(s) and/or settler(s)or would only be able to do so with the input of a significant amount ofenergy, effort, and/or cost.

Suitable flocculants generally have molecular weights in excess of1,000,000 and often in excess of 5,000,000. The flocculants may beanionic, cationic and/or zwitterionic. The dose of flocculant depends onthe properties of the slurry being treated and can be empiricallydetermined by one of skill in the art. In general, the flocculantpolymer dose required depends on both the nature of the feed slurry(settler or washer) and the type of flocculant used. However, typicallydose rates are generally within the range of 10-500 g/T based on polymersolids, per ton of red mud and more preferably 30-400 g/T.

In at least one embodiment the flocculant is selected from the groupconsisting of: (i) homopolymers of acrylic acid, (ii) copolymers ofacrylic acid and acrylamide, (iii) copolymers of acrylic acid andacrylamide modified to contain a hydroxamic acid moiety; and (iv)copolymers of acrylic acid and acrylamide modified to contain andcomprise ammonium acrylate. In at least one embodiment the flocculanthas a molecular weight greater than 10 million.

Similarly, while anionic polymer flocculants may be formed using anionicmonomers, it is also possible to modify certain nonionic vinyl additionpolymers to form anionically charged polymers. Polymers of this typeinclude, for example, those prepared by the hydrolysis ofpolyacrylamide.

The flocculant may be prepared in the solid form, as an aqueoussolution, as a water-in-oil emulsion, or as a dispersion in water.Representative anionic polymers include copolymers of acrylamide withsodium acrylate and/or 2-acryiamido 2-methylpropane sulfonic acid (AMPS)or an acrylamide homopolymer that has been hydrolyzed to convert aportion of the acrylamide groups to acrylic acid.

In at least one embodiment the polysaccharide is cross-linked. The crosslinking may be achieved with a crosslinking agent. In at least oneembodiment the cross-linking is accomplished by an the interaction witha cross linking agent on an ethylenically unsaturated monomer eithercontaining at least two sites of ethylenic unsaturation or containingone site of ethylenic unsaturation and one site of a reactive group suchas an epoxide or an aldehyde. Representative Cross-Linking Agentsinclude N,N-methylenebisacrylamide, N,N-methylenebismethacrylamide,polyethylene glycol diacrylate, ethylene glycol dimethacrylate,diethylene glycol dimethacrylate, triethylene glycol dimethacrylate,polyethylene glycol dimethacrylate, polypropylene glycol dimethacrylate,N-vinyl acrylamide, divinyl benzene, triallyl ammonium salts, N-methylallylacrylamide, glycidyl acrylate, acrolein, methylolacrylamide,glyoxal, epihalohydrins, dialdehydes diglycidyl ethers, and the like,and any combination thereof. In at least one embodiment the crosslinkingagent is added to the cross-linked polymers at a dosage of from about0.0001 to about 10, preferably from about 0,0001 to about 0,2 weightpercent based on the weight of the polymers.

In at least one embodiment, upon flocculation of a primary settler feed,a clear liquor/mud interface will form, and will gradually settle,providing a clear liquor supernatant layer overlying a mud layer. Thelower mud layer contains the flocculated material. The overlyingsupernatant is the liquor that is separated for subsequent filtrationand contains only a minimal amount of mud solids. The inventiondiminishes the amount of suspended solids in such supernatant, and hencedecreases the extent of filtration required to obtain a given purity ofsodium aluminate solution.

In at least one embodiment, the solids captured by the combination ofthe flocculant and modified polysaccharide contain more of the mineralsand solids comprised of silica and potassium such that the solidsreporting to the overflow of the clarifier contain a lower amount ofsuch materials, resulting in selective flocculation and settling of suchmaterials.

In at least one embodiment, the polymeric flocculant and thepolysaccharide are both added to the primary settler feed as aqueoussolutions to facilitate rapid dispersion of each agent within theprimary settler feed.

In at least one embodiment the method Is conducted according to and/orin conjunction with some or all of one or more of the: compositions,apparatuses, and methods used in the Bayer Process described in one ormore of: U.S. Pat. Nos. 6,365,116, 6,726,845, 5,217,620, 5,478,477,5,387,405, International Patent Application WO 99/29626, and scientificpapers: Step change improvements in underflow rheology by Berger A, etal, Proceedings of the 14th International Seminar on Paste and ThickenedTailings, pp. 135-141 (2011), and Effect of Surfactants on BauxiteResidues Suspensions Viscosity by Frost, R, et al, Colloids and SurfacesA; Physicochemical and Engineering Aspects, 292(1), pp. 21-26 (2007).

EXAMPLES

The foregoing may be better understood by reference to the followingexamples, which are presented for purposes of illustration and are notintended to limit the scope of the invention.

Red mud settling tests were performed in order to evaluate the effect onliquor overflow clarity and settling rate with and without addition of across-linked polysaccharide to a standard anionic flocculant treatment.These tests involved mixing a known amount of flocculant solution (andpolysaccharide where appropriate) into a cylinder containing Bayerprocess red mud slurry. After mixing, the cylinders were left to settleand after a given period of time the liquor at the surface of thecylinder was sampled, filtered and the filtered residue weighed. Theclarity of the liquor is described in terms of overflow solids withunits of gram per litre (essentially the lower the overflow solids themore effective the chemical treatment strategy).

Example 1

Samples of plant settler feed slurry were treated using a 0.1% solutionof a commercially available 100% anionic latex flocculant and a 13%solution of a modified (cross-linked) polysaccharide. In this case themodified polysaccharide was dextran. Table 1 details the overflow solidsfor red mud slurry flocculated with (i) anionic flocculant and (ii) acombination of anionic flocculant and the cross-linked polysaccharide,Addition of modified polysaccharide to the slurry, at a dose rate in theorder of 30 ppm, resulted in approximately 50% reduction in overflowsolids. A substantial reduction in overflow solids was achieved over theflocculant dose range of 40-130 g/T.

TABLE 1 Overflow solids measured from red mud settling tests using astandard anionic flocculant with and without additional treatment withmodified polysaccharide. Dose Anionic Flocc Dose MPS Overflow SolidsTreatment (g/T) (ppm active) (g/L) AF 43 0 1.28 AF + MPS 43 3.6 0.54 AF85 0 1.19 AF + MPS 85 4.2 0.45 AF 128 0 0.71 AF + MPS 128 4.4 0.44 AF =Anionic Flocculant MPS = Modified polysaccharide (cross-linked dextran)

Example 2

The method of application of the modified polysaccharide was assessed byaddition of a modified polysaccharide prior to, with or after theflocculant addition. The anionic flocculant and modified polysaccharidewere the same as those used in example 1 as was the test method. Slurryused was again plant settler feed, Table 2 details the overflow solidswhen the modified polysaccharide was pre-dosed, and co-dosed with theanionic flocculant in one set of tests and when it was co-dosed andpost-dosed with the anionic flocculant in a second set of tests. Themodified polysaccharide was effective in all methods of application,reducing the overflow solids when compared to the same dose of anionicflocculant applied without additional modified polysaccharide.

TABLE 2 Overflow data from settling tests comparing the method ofapplication of a modified polysaccharide when applied with aconventional anionic flocculant. Dose Anionic Flocc Dose MPS MPS addedrelative Overflow (g/T) (ppm active) to flocculant Solids (g/L) 73 0 —0.63 73 5.7 Pre-dose 0.29 73 5.7 Co-dose 0.31 73 0 — 0.69 73 5.7Post-dose 0.32 73 5.7 Co-dose 0.37

Example 3

The use of polysaccharides such as dextran has previously beenidentified and used in combination with anionic flocculants as a clarityaid in red mud settlers (as described, for example, in U.S. Pat. No.3,085,853). Red mud settling tests, as described above, were conductedto assess the relative efficacy of a modified (cross-linked)polysaccharide compared to the same unmodified polysaccharide. Theanionic flocculant and modified polysaccharide were the same as thoseused in example 1. The modified and unmodified polysaccharide solutionsscontained the same amount of polysaccharide in each treatment. Table 3details the overflow solids of red mud treated with a combination of (i)anionic flocculant and cross-linked dextran and (ii) anionic flocculantwith dextran. In this example a commercially available, 100% anionicflocculant was again used but it was a different product to that used inexamples 1 and 2.

TABLE 3 Overflow solids from red mud testing with modifiedpolysaccharide and unmodified polysaccharide combinations with anionicflocculant. Anionic Polysaccharide Red Mud Treatment Flocculant Dose(ppm Overflow Regime Dose (g/T) active) Solids (g/L) Anionic Flocculant75 0 0.43 Anionic Flocculant + 75 0.60 0.34 Polysaccharide 1.20 0.33Anionic Flocculant + 75 0.78 0.31 Modified 1.56 0.29 PolysaccharideThe data demonstrates that addition of modified polysaccharide issurprisingly more effective at reducing overflow solids compared withthe use of unmodified polysaccharide.

Example 4

Analysis of the solids obtained from settling tests conducted asdescribed in the previous examples was completed to determine the natureof the solids removed by the application of modified polysaccharide. Redmud settling tests were completed using a combination of commerciallyavailable hydroxamated anionic flocculant both with and withoutadditional application of a cross linked polysaccharide. The treatmentand overflow solids obtained is listed in table 4.

TABLE 4 Overflow solids from red mud settling tests using a hydroxamatedanionic flocculant and modified polysaccharide. Dose Anionic FlocculantDose MPS Overflow Solids (g/T) (ppm active) (mg/L) 150 0 112 150 1 72150 2 62 150 3.5 45Additionally, samples of the overflow solids were collected, dried andsubjected to X-ray fluorescence spectroscopy (XRF) to determine theelemental components present in the overflow. Table 5 shows the amountand relative change in a range of elements analyzed in the treatedsamples. Surprisingly treatment with the cross-linked polysaccharideresults in significantly less silica and potassium containing materialsreporting in the overflow solids. When compared to the reduction in therelative amounts of other elements such as iron, titanium and calcium,there is a lot less silica and potassium in the treated samples. Thisindicates that the modified polysaccharide is selective in removinginsoluble minerals which may be high in silica and potassium content.Examples of such minerals would be muscovite or mica.

TABLE 5 Concentration of product (expressed as ppm or mg of analyte perlitre of overflow) % Reduction in component from MPS treatment FloccFlocc + Flocc + Flocc + (3.5 ppm) vs treatment MPS MPS MPS Flocc onlyAnalyte only (1 ppm) (2 ppm) (3.5 ppm) treatment (%) Fe₂O₃ 39.3 31.425.4 19.4 50.6 K₂O 2.69 1.75 1.28 0.81 69.9 SiO₂ 18.6 12.5 9.7 5.8 68.8CaO 1.34 1.08 0.90 0.73 45.5 TiO₂ 5.65 4.70 3.89 3.09 45.4

Example 5

Raked settling tests conducted in an Imhoff cone were performed toevaluate the effect of a flocculant/modified polysaccharide combinationon the flow characteristics and rheology of flocculated red mud. Thesetests involved mixing one litre of Bayer plant final washer feed red mudslurry together with a known amount of conventional flocculant solutionand modified polysaccharide solution in a cylinder. After mixing, theslurry was Immediately transferred to an Imhoff cone and the slurry wasthen raked to a specific mud bed volume (bed underflow density wascalculated from the bed volume and known feed solids concentration). TheImhoff cone plug was then released and the time taken tor the thickenedslurry to discharge from the cone was measured. The time required fordischarge of the entire slurry is a measure of mud flow-ability (orrheology). This discharge time was compared with feed slurry treatedwith flocculant alone (the ‘normal’ process for the settler and washercircuit). The faster the discharge rate the better the mud underflowrheology and ultimately properties for pumping from the thickenerunderflow.

Table 6 details the average discharge rate of the thickened red mudslurry from the raked Imhoff cone after treatment with flocculant andafter treatment with the combination of flocculant and modifiedpolysaccharide. The flocculant dose was kept constant at 100 g/T whilethe dose of modified polysaccharide (when applied) was 3.75 ppm. In thiscase, the modified polysaccharide used was a cross-linked dextran. Theflocculant used was a conventional polyacrylate/polyacrylamideflocculant commercially available and typically used for red mudsettling in washers.

TABLE 6 Average time taken to discharge consolidated red mud slurry froman Imhoff cone across a range of average bed underflow densities.Flocculant dose was constant at 100 g/T. Polysaccharide dose was 3.75ppm. Average Underflow Average Treatment Density Discharge TimeDischarge Regime (g/100 mL) Recorded (sec) Rate (mL/sec) Flocculant 20.010 100 21.3 60 17 21.4 >300 0 22.9 >300 0 25.0 >300 0 34.0 >300 0Flocculant + 21.3 10 100 Modified 22.5 10 100 Polysaccharide 25.0 10 100

Raking of slurry treated with conventional flocculant to an averageunderflow density of greater than 21%, resulted In effectively no flowwhen the plug was removed. Effectively the slurry did not discharge fromthe Imhoff cone due to the poor flow characteristics of the raked,consolidated mud at the base of the cone.

This was compared to the results obtained after raking slurry that wastreated with the same dose of conventional flocculant together withmodified polysaccharide (3.75 ppm). Across a similar range of highaverage underflow densities, slurry discharged from the cone withoutrestriction (˜100 mL/sec). This is a significant increase in the flowproperties of the red mud when compared to the control test (flocculantonly treatment).

In a separate test mud was treated in a similar manner (the sameflocculant at the same dose) but the modified polysaccharide dose wasreduced to 0.75 ppm, When consolidated to an average underflow densityof 23.8 /100 mL a discharge time of 19 seconds was recorded (averagedischarge rate of 53 ml/sec). Comparing this to the data from Table 1,consolidation of mud to this concentration without addition of modifiedpolysaccharide would be expected to result in no flow, while a higherdose of modified polysaccharide (3.75) clearly results in substantiallyhigher flow rates.

The results of these tests indicate that addition of flocculant andpolysaccharide in combination to a red mud washer circuit or primarysettler would enable the targeted underflow density to be substantiallyincreased while fluid characteristics of the underflow solids could bemaintained or enhanced.

While this invention may be embodied in many different forms, there aredescribed in detail herein specific preferred embodiments of theinvention. The present disclosure is an exemplification of theprinciples of the invention and is not intended to limit the inventionto the particular embodiments illustrated. All patents, patentapplications, scientific papers, and any other referenced materialsmentioned herein are incorporated by reference in their entirety.Furthermore, the invention encompasses any possible combination of someor all of the various embodiments described herein and/or incorporatedherein. In addition the invention encompasses any possible combinationthat also specifically excludes any one or some of the variousembodiments described herein and/or incorporated herein.

The above disclosure is intended to be illustrative and not exhaustive.This description will suggest many variations and alternatives to one ofordinary skill in this art. All these alternatives and variations areintended to be included within the scope of the claims where the term“comprising” means “including, but not limited to”. Those familiar withthe art may recognize other equivalents to the specific embodimentsdescribed herein which equivalents are also intended to be encompassedby the claims.

All ranges and parameters disclosed herein are understood to encompassany and all subranges subsumed therein, and every number between theendpoints. For example, a stated range of “1 to 10” should be consideredto include any and all subranges between (and inclusive of) the minimumvalue of 1 and the maximum value of 10; that is, all subranges beginningwith a minimum value of 1 or more, (e.g. 1 to 6.1), and ending with amaximum value of 10 or less, (e.g. 2.3 to 9.4, 3 to 8, 4 to 7), andfinally to each number 1, 2,3, 4, 5, 6, 7, 8, 9, and 10 contained withinthe range. All percentages, ratios and proportions herein are by weightunless otherwise specified.

This completes the description of the preferred and alternateembodiments of the invention, Those skilled in the art may recognizeother equivalents to the specific embodiment described herein whichequivalents are intended to be encompassed by the claims attachedhereto.

1. A method for treating Bayer Process liquor containing red mudcomprising: (a) adding to the slurry a flocculant and a cross-linkedpolysaccharide and (b) removing flocculated red mud contained in theBayer process liquor by at least one of: sedimentation, centrifugation,filtration the use of a washer, and the use of a settler.
 2. The methodof claim 1 in which the cross-linked polysaccharide is added to theslurry prior to the addition of the flocculant.
 3. The method of claim 1in which the cross-linked polysaccharide is added to the slurry afterthe addition of the flocculant.
 4. The method of claim 1 in which thecross-linked polysaccharide is added to the liquor at the same time asthe addition of the flocculant.
 5. The method of claim 1 in which thecross-linked polysaccharide and the flocculant are added to liquorcontaining red mud which is primary settler feed or feed slurry of a redmud washer.
 6. The method of claim 5 further comprising the step ofincreasing the density of the flocculated red mud to an amount such thatbut for the presence of the flocculant with the cross-linkedpolysaccharide, the flocculated red mud would have a rheology that wouldprevent its free flow as an underflow through a settler or washer. 7.The method of claim 1 in which the flocculant is selected from the listconsisting of: (i) homopolymers of acrylic acid, (ii) copolymers ofacrylic acid and acrylamide, (iii) copolymers of acrylic acid andacrylamide modified to contain a hydroxamic acid moiety, and (iv)copolymers of acrylic acid and acrylamide modified to contain ancomprises ammonium acrylate, and (v) any combination thereof.
 8. Themethod of claim 1 in which the cross-linked polysaccharide comprises twoor more different types of polysaccharide polymers.
 9. The method ofclaim 1 in which the cross-linked polysaccharide is cross-linkeddextran.
 10. The method of claim 1 in which the flocculant has amolecular weight of greater than 10 million.
 11. The method of claim 1in which the method further comprises the step of raking the liquorafter the addition of the flocculant and a cross-linked polysaccharide.12. The method of claim 1 comprising the step of reducing the overflowsolids in a clarifying vessel.
 13. The method of claim I comprising thestep of selectively reducing silicon and potassium containing mineralsin the overflow solids.